MnOx/catechol/H2O: A cooperative catalytic system for aerobic oxidative dehydrogenation of N-heterocycles at room temperature

Article abstract of DOI:10.1016/j.tetlet.2019.151425

Amorphous manganese oxide doped by Na⁺ ion (Na-AMO) was successfully prepared and found to be an efficient heterogeneous catalyst in aerobic oxidative dehydrogenation of N-heterocycles, cooperate with catechol. Na-AMO was fully characterized by XRD, XPS BET H₂-TPR, CO₂-TPD FT-IR, TEM, SEM and had rich amounts of surface absorbed active oxygen species which are responsible for superior catalytic performance. The synergistic interaction between Na-AMO and catechol makes catalytic system efficient and tolerant, which offers various N-heterocycles in good to excellent yields under mild conditions.

Full text of DOI:10.1016/j.tetlet.2019.151425

Journal Pre-proofs
MnOx/catechol/H2O: a cooperative catalytic system for aerobic oxidative de-
hydrogenation of N-heterocycles at room temperature
Tao Tang, Xiuru Bi, Xu Meng, Gexin Chen, Mingxia Gou, Xiang Liu, Peiqing
Zhao
PII:
S0040-4039(19)31216-X
https://doi.org/10.1016/j.tetlet.2019.151425
TETL 151425
DOI:
Reference:
Tetrahedron Letters
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Accepted Date:
29 September 2019
13 November 2019
19 November 2019
Please cite this article as: Tang, T., Bi, X., Meng, X., Chen, G., Gou, M., Liu, X., Zhao, P., MnOx/catechol/H2O: a
cooperative catalytic system for aerobic oxidative dehydrogenation of N-heterocycles at room temperature,
Tetrahedron Letters (2019), doi: https://doi.org/10.1016/j.tetlet.2019.151425
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MnOx/catechol/H2O: a cooperative catalytic system
for aerobic oxidative dehydrogenation of
N-heterocycles at room temperature
Tao Tang,a,b Xiuru Bi,a,b Xu Meng,*,a Gexin Chen,a Mingxia Gou,a Xiang
Liu,c Peiqing Zhao*,a
a. State Key Laboratory for Oxo Synthesis and Selective Oxidation,
Suzhou Research Institute of LICP, Lanzhou Institute of Chemical
Physics (LICP), Chinese Academy of Sciences, Lanzhou 730000, China.
Fax:
+969318277008;
Tel:+86931
4968688;
E-mail:
xumeng@licp.cas.cn, zhaopq@licp.cas.cn
b．University of Chinese Academy of Sciences, Beijing 100049, China
c．College of Materials and Chemical Engineering, Key Laboratory of
Inorganic Nonmetallic Crystalline and Energy Conversion Materials,
China Three Gorges University, Yichang, Hubei 443002, P. R. of China
Keywords: manganese oxide, recyclable catalyst, oxidative
dehydrogenation, cooperative catalysis
Graphic abstract:
Abstract: Amorphous manganese oxide doped by Na+ ion (Na-AMO)
was successfully prepared and found to be an efficient heterogeneous
catalyst in aerobic oxidative dehydrogenation of N-heterocycles,
cooperate with catachol. Na-AMO was fully characterized by XRD, XPS

BET H2-TPR, CO2-TPD FT-IR, TEM, SEM and had rich amounts of
surface absorbed active oxygen species which are responsible for superior
catalytic performance. The synergistic interaction between Na-AMO and
catechol makes catalytic system efficient and tolerant, which offers
various N-heterocycles in good to excellent yields under mild conditions.
1. Introduction
Unsaturated N-containing heterocycles are key structural motifs in
many pharmaceutical and bioactive compounds.[1] Besides, they are
important intermediates in organic synthesis for providing late-stage
functionalized products.[2] Many processes have been developed for the
synthesis of N-heterocycles liking quinoline, quinazoline, indole, pyridine
and cridine, mainly include intermolecular cyclization and intramolecular
dehydrogenation.[3-7] Gradually, oxidative dehydrogenation of these
N-heterocycles has developed as an important and efficient pathway. In
recent years, homogeneous catalysts and heterogeneous catalysts both
were developed, while photoredox catalysis and electrochemical process
were proved to be successful means. [8-18] Especially, Beller and Stahl
with their co-workers respectively prepared novel iron- and cobalt-based
catalysts for aerobic oxidative dehydrogenation of nitrogen heterocycles
with good yields.[19] Very recently, Shi`s group synthesized a metal-free
single-active site heterogeneous catalyst via oxidative polymerization of
maleimide derivatives for selective oxidation of N-containing
heterocycles. [20] Although these catalytic systems show merits of
heterogeneous catalysis, toxic organic solvents, high reaction
temperature. Therefore, catalytic systems employing non-noble
recyclable metal catalyst, green reaction media and mild reaction
conditions are desirable. Amorphous manganese oxide (AMO) composed
by disordered [MnO6] units has been applied in electrocatalysis,
environmental catalysis and clean organic synthesis because of its
relatively large surface area, increased numbers of active sites and rich
amounts of surface oxygen species compared with crystalline MnO2.
[21-24] So far, aerobic oxidative dehydrogenation of N-heterocycles
catalyzed by Mn-based materials is less explored. For instance, Suib`s
group reported a mesoporous MnOx-catalyzed oxidative dehydrogenation
of N-heterocycles in DMF and they found that AMO was less efficient
for the oxidative dehydrogenation. [25] From the viewpoint of a green

and environmental perspective, design and development of MnOx-based
catalytic system being able to proceed oxidative dehydrogenation of
N-heterocycles under mild conditions in green solvent are highly
desirable.
According to the work from Stahl, Kobayashi and Doris, bioinspired
metalloenzyme-like cooperative systems involving a transition metal and
a suited catechol derivative could generate a synergistic effect which
conducted oxidative dehydrogenation of N-heterocycles under ambient
conditions. [26-29] It was reported that catechol molecules or oxidized
counterparts (o-quinone) played a role as redox-active organic catalysts
and transition-metal-based materials promoted the in-situ reoxidation of
hydroquinones to finish the catalytic cycle. Interestingly, we recently
found that Na-modified AMO (designated as Na-AMO) could efficiently
catalyze catechol Q-2 to quinone Q-22 (table1,Q-22) since we
demonstrated manganese oxide octahedral molecular sieve (OMS-2) had
catalytic activity for the oxidation of catechols for the synthesis of
benzoxazoles.[30] These results indicated there was a possibility that
AMO-based material and a suitable catechol compound could make up a
cooperative co-catalytic system for oxidative dehydrogenation of
N-heterocycles under mild conditions.
In continuation of our previous research for application of
MnOx-based materials in oxidative catalytic system, [31] we would like
to report a cooperative catalytic system comprised by Na-doped AMO
and 4-tert-butylcatechol for oxidative dehydrogenation of N-heterocycles.
The co-catalytic process tolerated a wide range of N-heterocycles and
could carry out the reactions in H2O as the solvent at room temperature.
2. Experimental
2.1 Catalyst Preparation
Na-AMO. Firstly, KMnO4 (5.89 g, 37.27mmol) was dissolved into 100
mL of deionized water and the solution was stirred until KMnO4 was
fully dissolved at slightly elevated temperature less than 50 ℃ if
necessary. Then, MnSO4 ·H2O (8.8 g, 52.06mmol) and NaNO3 (1.55 g,
18.23mmol) were dissolved into 30 mL of deionized water. The KMnO4
solution was dropwise added into MnSO4 solution at room temperature

and the resulted mixed solution was kept stirring for another 10 minutes.
Finally, the black suspension liquid was filtrated and washed. The filter
cake was dried at 150℃ under air for 12 h. Eventually, black power was
obtained as the target catalyst Na-AMO.
2.2 The general procedure of oxidative dehydrogenation
N-heterocycle substrate (0.2 mmol), Na-AMO (20 mg, 10 mol%),
catechol Q-2 (3.3 mg, 0.02 mmol) was added into a reaction tube. The
reaction tube was filled and protected with oxygen using a O2 balloon,
then H2O was added into the reaction tube with a syringe. The reaction
was allowed to react at room temperature for 20 hours. After completion
of reaction, the resulting mixture was filtered, washed with EtOAc and
extracted by EtOAc. The organic phases were combined, washed with
brine and dried with Na2SO4. Next, organic solution was concentrated
under reduced vacuum to give the crude product. Finally, the product was
purified by silica gel chromatography to yield the pure product。
3. Results and discussion
We combined substituted catechols or quinone compounds (Q-1 to
Q-6,Q-22) and MnOx to test the catalytic activity. As expected, AMO
with Q-1 showed a little catalytic activity in the reaction in EtOH when
air was used as the oxidant (Table 1, entry 1). According the examination
of substituted catechols Q-1 to Q-5 and 1,10-phenanthroline-5,6-dione
Q-6, it was found that the co-catalysts affected the reaction obviously and
Q-2 was proved to be the most suitable one in the cooperative catalytic
system (Table 1, entries 1-6). After that, manganese oxide–based
materials were investigated to further improve the reaction, which
showed that manganese oxide octahedral molecular sieves (OMS-2), its
H-modified counterpart (H-OMS-2) and commercial bulk MnO2 all did
not offer better results of the reactions (Table 1, entries 7-9).
Interestingly, Na-doped AMO using NaNO3 as the dopant provided 76%
yield of 2a (Table 1, entry 10). Furthermore, it was found that both
oxygen and hydrogen peroxide as the green oxidant could improve the
yield of 2a to 95%, (Table 1, entries 11 and 12). Compared to O2, H2O2
has the disadvantage of handing hazard, so we chose molecular oxygen as
environmentally acceptable oxidant. At the same time, the reaction was

proved to be aerobic because the poor result was obtained when the
reaction was carried out under N2 (table1 entry 11). Subsequently, many
solvents including biomass-derived dimethyl carbonate (DMC), DMF,
dichloromethane (DCM), toluene and the greenest water, were tested in
the reaction and the results showed that the reaction could occur smoothly
in H2O with complete conversion and nearly quantitative yield (Table 1,
entries 13-17).
Further optimization demonstrated that decreasing the reaction time
slightly lowered the yields of the reaction (table1 entry18). Then we
changed the ratio of Na-AMO/Q-2 and found that the yields was
susceptible to the amount of Q-2(table1 entry 13, 19 and 20,23). in
addition, the changes in Na-AMO had slightly impact to the yields (table1
13, 21 and 22,23). Na-AMO (10mol%) and Q-22(5mol%) seemed best
condition(table1 19), but a poor yield was attained in gram scale reaction
(table1 entry 24). So we chose entry 13 (table1) as optimal reaction
condition. As a result, the environmentally-friendly catalytic system for
the oxidative dehydrogenation of 1a was established as following:
Na-AMO as the catalyst, catechol Q-2 as the co-catalyst, O2 as the
oxidant and H2O as the reaction media.
Table 1 optimization of reaction condition
AMO (10 mol%)
co-catalyst (10 mol%)
EtOH, air, r.t., 20 h
N
H
N
1a
2a
-Bu
OH
OH
t
-Bu
OH
OH
t
-Bu
OH
OH
OH
t
Me
OH
OH
-Bu
Q-3
t
Q-2
Q-22
Q-1
O
O
N
O
O
OH
OH
OH
Ph
Ph
OH
N
Q-5
Q-4
Q-6
Entry
1
Catalyst(mg)
AMO
Co-catalyst(mol%) Solvent Oxidant Yield (%)
b
EtOH
air
Q-1
19

2
3
4
5
6
7
AMO
AMO
AMO
AMO
AMO
OMS-2
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
air
air
air
air
air
air
Q-2
Q-3
Q-4
Q-5
Q-6
67
31
8
56
14
58
Q-2
Q-2
Q-2
Q-2
Q-2
Q-2
Q-2
Q-2
Q-2
8
9
H-OMS-2
a-MnO2
EtOH
EtOH
EtOH
EtOH
EtOH
DCM
DMC
DMF
air
air
41
57
10
11
12d
13
14
15
Na-AMO
Na-AMO
Na-AMO
Na-AMO
Na-AMO
Na-AMO
air
76
O2
95 (35c)
95
H2O2
O2
95
O2
34
O2
35
16
17
18e
19
20
21
22
23
24f
Na-AMO
Na-AMO
Q-2
Q-2
PhMe
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
O2
O2
O2
O2
O2
O2
O2
O2
O2
87
98
85
95
83
94
95
85
70
Na-AMO
Q-2
Na-AMO
Q-2(5)
Q-2(12.5)
Q-2
Na-AMO
Na-AMO(10)
Na-AMO(30)
Na-AMO(10)
Na-AMO
Q-2
Q-2(5)
Q-2(5)
25
26
27
28
29
30
-
-
-
-
H2O
H2O
H2O
H2O
H2O
H2O
O2
O2
O2
O2
O2
O2
N.R.
5
8
8
56
95
Q-2(10)
Q-2(100)
Na-AMO (20)
Na-AMO (100)
Na-AMO (20)
-
-
Q-22 (10)
a.Reaction conditios: 1a (0.2 mmol), MnOx (20 mg, 10 mol%), co-catalyst
(10 mol%), solvent (1 mL), 20 h, oxidant, room temperature.
1
b.Yields were determined by HNMR using dibromomethane as theinternal
standard.
c. Under N2.
d. 10 uL of H2O2 (30 wt%) was used.
e. For 10 h.

f: reaction was magnifiied by 100 times; Q-2 (5 mol%)
Na-AMO was proved as best heterogeneous catalyst, which was
characterized by XRD, XPS, BET, SEM, TEM, FT-IR, H2-TPR,
CO2-TPD. First of all, XRD patterns shows that Na-AMO has the same
crystal phase to pristine AMO featured typical amorphous structure,
composed by disordered [MnO6] units as (Fig. 1). ICP-MS results of
Na-AMO indicate that K+ ion of Na-AMO was replaced partially by Na+
ion, which means Na+ doping was successful. Subsequently, the
morphologies of the catalysts were probed by scanning electron
microscope and transmission electron microscope.( Fig. S4, S5)
Figure 1. XRD patters of original AMO, Na-AMO and used Na-AMO.
For getting more information of surface chemical properties of the
catalysts, X-ray photoelectron spectroscopy was carried out. As expected,
O 1s of pristine AMO could be deconvoluted into two peaks that are
surface adsorbed oxygen species (531.6eV) with low coordination and
lattice oxygen (529.8eV) for maintaining the catalyst structure, which is
agreement with the results of the reference.[24] For O1s of Na-AMO,
three kinds of oxygen species (532.9eV, 531.2eV, 529.5eV) These results
exhibit that Na-AMO has more amounts of surface oxygen vacancies than
AMO does, because surface active oxygen species generally adsorb on
the oxygen vacancies (Fig. 2 and Table S1). Accordingly, more
abundant of oxygen vacancies of Na-AMO possibly result into superior
catalytic activity. On the other hand, Mn 2p of AMO and Na-AMO was
analyzed by XPS and they show similar results with three oxidation stage
(Mn2+,Mn3+,Mn4+) (Fig. S3 and Table S2). To further examine the
catalyst composition, FT-IR was used to characterize Na-AMO, as
showed in Fig 3. The peaks at 550 cm-1 and 720 cm-1 were attributed to

Mn-O and Mn-O-Mn stretching vibration. The peaks during
3200-3500cm-1 were assigned to stretching vibration of H2O and OH- and
the peak of 1600 cm-1 aroused from bending vibration of H2O and OH-.
Abundant surface OH- and H2O may derive from the break of Mn-O-Mn
chain of Na-AMO.[32]
Figure 2. XPS spectra of O 1s for original AMO and Na-AMO.
Figure 3 FT-IR spectrum of Na-AMO
Redox characterization of matarial is crucial For our redox reaction, so
we carried out H2-TPR exeperiment Fig 4(a), there are two peaks located
in 294℃ and 380℃ respectively, which means two change of oxidation
state may exist in our reaction. The peak of 294℃ could attribute to the
trasformation of MnO2 to Mn2O3, and the peak of 380 ℃ could assign to
Mn2O3 to MnO.[32] Additionally, the alkalinity of catalyst is helpful to
dehydrogenation, so we used CO2-TPR to evaluate the Alkaline site as
showed in Fig 4(b) which included two wide peaks located at 360℃ and
460℃, and a narrow peak in 546℃. CO2-TPR means Na-AMO is fulled

with Alkaline site which may resulted from introduction of Na+.
Figure 4. H2-TPR spectrum of Na-AMO (a); CO2-TPD spectrum of Na-AMO (b);
We conducted hot filtration experiment to ensure the true catalyst is
Na-AMO itself rather than leached Na+ and Mnx+ ion. we performed the
reaction with tetrahydroquinolines as substrate, Under stardand condition.
After one hour, the catalyst was filtrated, and the NMR yield of
quinolines was 63%. At the same time, the filtrate was kept reacting for
another 19 hour, and no further improvement in yield was detected.
Meanwhile, no ion leached was observed when the filtrate analyzed by
ICP-MS, which means Na-AMO is completely heterogeneous. Recycling
test shows Na-AMO could be reused for at least 8 consecutive times with
slightly decreasing yields and the yield was stable at around 87%
eventually as show in Fig. 5.
Figure 5. The recycling experiments.
With the cooperative catalytic system containing as-synthesized
Na-AMO and 4-tert-butylcatechol Q-2 in hand, a number of N-containing
substrates were examined. As shown in Table 2, substituted

tetrahydroquinolines could be tolerated in H2O and good to excellent
yields of corresponding quinolines were obtained under the present
reaction conditions (Table2, 2a-2i). Importantly, bio-active
2,3-dihydro-1H-cyclopenta[c]quinoline 2i can be catalyzed by the
cooperative Na-AMO/catechol system, which enhances the practicality of
the catalytic system. Moreover, substituted quinazolines were obtained in
good yields ranging from 60% to 80% by Na-AMO/Q-2 through
oxidative dehydrogenation of tetrahydroquinazolines in water (Table 2,
2j-2p). Tetrahydroquinazolines with heteroatom-containing substitute and
aliphatic substitute both proceeded very well under the standard
conditions. Subsequently, various indolines and 2,3-dihydro-7-azaindole
were tested in the reactions (Table 2). Indole and 2-methyl-indole(table2
2r, 2t) were obtained in moderate yields, while 7-azaindole and
2-carboxyindole were isolated in 80% and 90% yields
respectively.(table2 2q 2s) More complex tetrahydro-β-carbolines which
contain indoline as the main structure also carried out the reactions
smoothly, although the yields of desired products were slightly low
(Table 2, 2u,2v). In the case of oxidative dehydrogenation of Hantzsch
ester compounds, it was found that the reactions hardly occurred at room
temperature in pure water. Delightedly, the reactions could work in mixed
solvent of EtOH and H2O (v/v = 1/10) when the reaction temperature was
increased to 40 ℃ , and moderate to excellent yields of desired
substituted pyridines were offered (Table 2w-2y). Eventually, polycyclic
compound 2z was obtained, which is the key building block for the
synthesis of a number of biologically active molecules, such as
antiprotozoal agent and phase II topoisomerase inhibitor TAS-103 (Table
2, 2z).[33-34] For tetrahydroisoquinoline, it was found that the elevated
temperature was needed and an incomplete dehydrogenation product was
provided chemoselectively (Table 2, 2za). And, 9,10-dihedroacridine
could participate in water at room temperature and acridine was isolated
in 90% yield (Table 2, 2zb)
Table 2 The scope of substrate

Na-AMO (10mol%)
Q-2(10mol%)
N
H
H2O, O2, r.t., 20h
N
2a
1a
Cl
tBu
ph
N
N
N
N
O2N
N
N
2d 95%
2fb 71%
2cb
61%
2a 95%
2b 95%
2e 70%
Br
N
N
N
ph
N
N
N
N
O
2hb 65%
2j 80%
2k 79%
2gb 73%
2i 73%
N
N
N
N
N
N
N
N
O
Cl
2nb 70%
2ob 68%
2lb 70%
2mb 60%
N
N
COOH
N
N
N
N
N
F
2r 50%
2s 80%
2p 65%
2q 90%
2t 50%
OMe
O
ph
O
O
O
N
N
OMe
OMe
OMe
N
N
N
N
Cl
2xb,c
2wb,c 65%
95%
2v 70%
2u 70%
O
ph
N
O
O
EtO
OEt
N
N
N
2zab 83%
2zb
2yb,c 80%
2zb 90%
73%
a. reaction condition: a. Reaction conditions: 1 (0.2 mmol), Na-AMO (20 mg, 10 mol%), Q-2 (10 mol%), H2O
(1 mL) 20 h, r.t., O2 balloon, isolated yields.
b.40℃
c. EtOH/H2O(v/v)=1/10.40℃

To gain insight into the reaction mechanism, some control experiments
were performed. Firstly, the oxidative dehydrogenation of 1a did not
occur at all without manganese oxide and catechol Q-2 (Table 1, entry
25). Then, it was found that trace amounts of desired product 2a were
observed when 0.1 equiv. or 1.0 equiv. of Q-2 was used in the absence of
manganese oxide (Table 1, entries 26-27). On the other hand, the blank
experiments proved that the reaction hardly occurred without Q-2 and O2
cannot efficiently re-oxidize the reacted Na-AMO that oxidizes 1a at
room temperature (Table 1, entry 28). And, the moderate yield of 2a was
detected using 100 mg of Na-AMO without Q-2 (Table 1, entry 29).
These findings tell that the catalytic specie is manganese oxide and
catechol does not play a role as the catalyst for the oxidative
dehydrogenation of 1a in our catalytic system. Moreover, the same yield
of 2a was offered when Q-2 was replaced by its oxidation state Q-22
under the standard conditions (Table1, entry 30). Considering results
from Table 1, we proposed that catechol Q-2 re-oxidizes reductive
Na-AMO and cooperates with Na-AMO as a redox couple.
4. Conclusion
In summary, we have developed an environmentally-friendly catalytic
system by the use of Na-doped AMO as heterogeneous catalyst, catechol
as co-catalyst, O2 as terminal oxidant and H2O as reaction medium for
oxidative dehydrogenation of various N-heterocycles. The highly
efficient catalytic system that works via the cooperative
Na-AMO/catechol redox couple tolerates wild range of substrates and
performs the reactions at room temperature. Employing the greenest
solution, mild reaction conditions and recyclable catalyst makes this
scalable catalytic method potential for industrial application, and we are
promoted to apply it in other clean synthesis processes.
Acknowledgements
We gratefully acknowledge the National Natural Science Foundation
of China (Grant Nos. 21573261, 21403256, 21805166) and the Youth
Innovation Promotion Association CAS (Grant No. 2018456).
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Graphic abstract:
Highlights
1. Heterogeneous oxidative dehydrogenation of N-heterocycles was
achieved in water.
2. Na-doped AMO combined with catechol was found to be an efficient
catalytic system.
3. A broad of N-heterocycles can be tolerated under the catalytic system.
4. The catalytic system shows merits of green chemistry
The authors declare no competing financial interest